This invention relates to magnetohydrodynamics (MHD); and more particularly to improved plasma electrodes for MHD generators.
MHD generators produce electrical power by motion of a high temperature electrically conductive gas through a magnetic field. This movement induces an electromotive force between opposed electrodes within the generator.
In order for an MHD generator to produce electrical power it is required that the working gas be a good electrical conductor. Combustion gases are often not at a high enough temperature to give the required electrical conductivity. Therefore, it is customary to "seed" the combustion gas with an easily ionized substance to elevate the gas electron concentration and thus improve the electrical conductivity. Unfortunately, the best seed materials are the alkali metals which are quite reactive as reducing agents. This, coupled with the high temperature combustion gas, has a corrosive effect on hot electrodes. Also, if the electrodes are cooled below the condensation temperature of the seed, electrode shorting results from condensation of the seed on electrode and insulator surfaces. Hence, it is an object of this invention to provide a structure for reducing these problems.
It has previously been suggested that a plasma jet be used to start conduction to a gas shielded electrode. For example, one structure for starting such conduction is described in U.S. Pat. No. 3,480,805 which issued to Yerrel on Nov. 25, 1969. Another such structure is described in French Pat. No. 1,330,199 to Andre Dubois et al. As in the instant case, those prior structures attempted to use a jet of ionized gas to connect to the MHD generator's main plasma stream in order to deliver the generator's output to a load. As indicated in a subsequent paper delivered by Andre Dubois et al at a 1968 MHD colloquium in Warsaw, however, those earlier structures do not appear to have been entirely successful. Hence, it is another object of this invention to provide a fully operative plasma jet electrode for an MHD system.
Prior MHD generators such as those described above have attempted to direct a plasma stream from a first electrode element through an orifice comprising part of a second or "auxiliary" electrode element; and the MHD generator's load was then connected across corresponding elements of associated electrodes located on opposite sides of the MHD duct. These prior art systems, however, do not appear to have been adequate over a sufficiently large range of the generated current flow. Consequently, it is yet another object of this invention to provide a plasma jet electrode assembly that provides satisfactory operation over a wide range of current flows.
It is another object of this invention to provide a structure and method for increasing the efficiency of an MHD generator using plasma jet electrodes and for increasing the life span of such electrodes when used in an MHD system.